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LED Current Question

Quick Question. Lets say I have an led that runs on 12v with a max current of 1200ma. Does that mean I CANT give it any more than 1200? or that it uses 1200? Another example, If I hook that led up to a car battery, Would that be bad??

Re: LED Current Question

Lemme guess. 10watt Ebay emitters.

Without current regulation it's all about total voltage. Assuming it's a 10watt chinese array these are basically a 3x3 array of 1-watt LEDs. Assuming it's a white or blue LED anything over 11 volts will eventually kill them. The only good thing about chinese LEDs is their inherently high forward voltages let's them take a lot of abuse.

However, since 12volt batteries are rarely 12volts a .5 volt swing makes a huge difference.

You're better off getting 3x 3watt LEDs and using a cheap DC LED driver.

Re: LED Current Question

Lol, No Ebay emitters. Was just theoritical numbers. I'm looking at doing a few different projects, each with many leds. I'm just a little confused on the power requirements. Another example. If I take a 3 up cree xp-g r5, which is at 9.0V Forward Voltage @ 350mA. it it telling me it will draw 350ma at 9v? or that I can only feed it 350ma? If I happened to connect it to a 5000ma power supply at 9v, would that be bad for the led? For some reason I have it stuck in my head that leds will take whatever you give it current wise untill failure. Is this making any sence?

Re: LED Current Question

generally means that is the maximum that you aren't supposed to go over. It is possible to go over that current, but the makers of the LED have deemed that it would do harm to the LED.

Directly hooking up the LED to a car battery would be bad without some sort of current regulation or at least voltage regulation.

If you send in a certain amount of current through an LED, the current will pass through the LED and the LED will consume some voltage. That voltage is called the Forward Voltage (Vf), and it varies, depending on how much current you send through the LED.
If you apply a certain voltage to the LED, it will draw a certain amount of current. Current and voltage affect each other.

Now, particular to LEDs, as the current rises, the Vf will rise as well. However, for a large current increase, the Vf only rises a tiny bit. This is why it is better to use current to regulate power to an LED than to use voltage to regulate power to the LED.

And if you are a graphical learner, check out the first graph on page 7 of this datasheet for a commonly used LED: http://www.cree.com/products/pdf/xlamp7090xr-e.pdf
You'll see the relationship between voltage and current is exponentially proportional, and that the better variable to try to control would be the current.

Re: LED Current Question

Gotcha, Thats what I was looking for. I'm used to what I would consider normal electronics that only pull what they need. Ie, Connecting a 500w light into a 1000w power supply does nothing. However it appears the with leds, both matter. So for say 5mm leds, The resistor I'm adding for 12v operation is also lowering the 1300 amps that is feeding it. I've always assumed it was voltage only. I'm pretty damn new to constructing led assemblies.

Pg. 7 shows that the incremental resistance of the LED at 1A is about 0.3 ohms so a slight change in voltage will cause a large change in current and brightness. Note that the overall LED resistance appears to be about 3.7 ohms.

Re: LED Current Question

In saablusters DEFT EDC the Cree XP-C is driven at 1400mA, Cree say the maximum current is 500mA.

The maximum current isn't a never exceed value. the manufacturer decides that they want the LED to last xxxx hours, then they put it on a heatsink (not a very good one compared to what us flashaholics might use) and the highest current it can be used at and still meet the target lifetime they call the maximum current.

If you have the LED heatsinked better than the manufacturers and don't mind the LED having a shorter life you can go over the maximum current.

Last edited by JacobJones; 10-27-2011 at 10:52 AM.
Reason: Said the same thing twice

I cannot pretend to feel impartial about tint colours. I rejoice with the brilliant ones and am genuinely sorry for the poor browns.
~Sir Winston Churchill

Re: LED Current Question

The resistor I'm adding for 12v operation is also lowering the 1300 amps

Resistors waste huge amounts of energy, and can get red hot when used with large current levels. I've set crayon size 20watt radioshack resistors on fire with 10/20watt LEDs and couple Ohm loads simply because I didn't have a current regulated supply laying around.

Another example. If I take a 3 up cree xp-g r5, which is at 9.0V Forward Voltage @ 350mA.

Correct. LED's will only pass as much current as Vf (forward voltage ) is going through them, not the entire current capacity of the supply. A Cree XP-G passes ~350mA at 3volts, so if you have three XP-Gs in series and feed them no more than 9volts the LEDs will not pass any more than 350mA regardless of how much current is available (unless they get extremely warm). I've built a lot of light rigs using this same principle with 18.5 volt laptop bricks and six LED's in series. Not ideal, but it works at low current levels. Given that DC 350mA current regulated supplies cost $1.00 I might as well use one and not have to worry about exact voltage.

However, not all fixed voltage supplies are really that fixed. Walwarts and common adapters typically have much higher voltage out-put at low loads, and this throws your math to the wind and will damage LEDs without a current regulator. Laptops bricks and switched mode supplies typically do have reliable voltage specs. Again, DC current regulators are too cheap to dismiss.

Batteries are of course not very stable in terms of voltage either. Their voltage won't jump around like a cheap Walwart will, but voltage will drop as it discharges which is why a regulator should also be used.

Re: LED Current Question

In general it's best to use constant current circuits to drive LEDs but constant voltage circuits can be made to work if they're well regulated, you can adjust the voltage to match the string of LEDs, the LEDs have adequate heatsinking, and you have series strings of at least a few LEDs. Generally, LED current at any given voltage changes with temperature, and to a much lesser extent with age. Good heatsinking mitigates the changes in current due to temperature swings. The more LEDs you put in series, the more the total forward voltage needs to change in absolute terms for any given change in current. Here's an example. Let's say an LED draws 350 mA at 3.0 volts, and 1000 mA at 3.2 volts. If you want to keep the current in a single LED within 10% of 350 mA, you'll need to regulate the voltage to within perhaps 0.02 volts. This is a pretty tight specification. On the other hand, if you put 5 LEDs in series, you only need to regulate within 0.1 volts. Still the same percentage error, but building a 15V ± 0.1V supply is easier than building a 3V ± 0.02V supply.